Various techniques exist in the prior art for finding the direction (AOA) of a radio frequency source. The most well known of these is probably what might be termed standard heterodyning techniques. In accordance with these techniques a heterodyne receiver is tuned for peak output, and may also provide a readout of the frequency in question. Direction finding can be accomplished through the use of a highly directional antenna. Such techniques are very time comsuming and clearly not possible in instances where the signal source is "hopping" or changing rapidly in frequency.
A more sophisticated prior art technique might be categorized as the Bragg cell acousto-optic technique. In this technique a laser beam is reflected from the surface of a surface acoustic wave device carrying an acoustic representation of the signal of interest and the angle of reflection, called the Bragg angle, is measured. This angle of reflection varies as a function of frequency and, if desired, can be used for frequency determination. The angle-of-arrival at the antennae of such a system is determined by measuring the time the incoming signal arrives at each antenna. In order to make this measurement with any significant degree of resolution the antennae must be separated by a substantial distance, thereby precluding the use of an airborne measurement platform and otherwise limiting the effective application and use of this technique. In addition, several measurements of a given signal are needed since an integration is performed over several cycles when this technique is used.
The U.S. Pat. No. 4,443,801 to D. R. Klose et al., issued Apr. 17, 1984, discloses apparatus which performs high resolution angle-of-arrival measurements on multiple signals of different frequency. The invention disclosed therein is of particular utility when the signal source, or sources, is "hopping" in frequency. The Klose et al patent utilizes a pair of SAW Chirp-Z transform circuits for determining the phase difference between radio frequency signals received at two closely spaced (1/2 to 2 wavelengths) antennae. The Klose et al patent allows one to detect a class of frequency hopping signals that hop over an extremely broad bandwidth, but it necessitates the use of a relatively coarse detection channel or "bin" width. The concept of the Klose et al patent is not suitable if the receiver apparatus is required to cover a large number of frequency bins (e.g..gtoreq.256) over a broad spectrum in which the spectrum samples for bins for each hop are required to be of very narrow bandwidth.